Process and apparatus for the recovery of heat and chemicals from pulp liquor



y 7, 1959 F. E. HUTTON 2,893,829

PROCESS AND APPARATUS FOR THE RECOVERY OF HEAT AND CHEMICALS FROM PULP LIQUOR Filed May 19. 1953 2 Sheets-Sheet .1

INVENTQR FrankEJ /utm ATTORNEY ly 7, 1959 F. E. HUTTON 2,893,829

PROCESS AND APPARATUSFORTHE RECOVERY OFHEAT AND CHEMICALS FROM PULP LIQUOR Filed May 19, 1953 2 Sheets-Sheet 2 FlG.2

INVENTOR Frank E 2 zzzzfon ATTORNEY United States Patent PROCESS AND APPARATUS FOR THE RECOVERY 'OF HEAT AND CHEMICALS FROM PULP LIQUOR Frank E. Hutton, Wyckolf, NJ., assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Application May '19, 1953, Serial No. 355,910

6 Claims. (Cl. 23-48 The present invention relates in general to the construction and operation of apparatus for the recovery of heat and chemicals from digestor residual liquor, known as black liquor, in soda and kraft pulp mills. More particularly, this invention relates to improvements in the construction and operation of apparatus of the character described whereby sublimation of chemical constituents in the inorganic ash residue is reduced without any decrease in furnace stability or overall thermal efficiency.

Black liquor recovery units of the character described generally comprise a vertically elongated water cooled furnace in which the pulp residual liquor is burned, and from which the heating gases pass successively in heat transfer relation with steam generating and superheating tubes, a feed water heater, air heater, and black liquor concentrating apparatus, before being discharged to a stack by an induced draft fan. A forced draft fan directs combustion air through the air heater and the preheated air is usually divided between sets of primary air and secondary air inlet ports. Heated black liquor normally at a solids concentration in the range of 55- 70% is sprayed into the furnace chamber either as a relatively coarse spray horizontally across the furnace for deposition on the remaining vertical walls or as a finer spray through a longer trajectory towards the furnace floor. In either case the liquor as been dehydrated to a substantially dry char on reaching the furnace floor, and a relatively thick bed of highly reactive char accumulates I on the inclined floor and is burned thereon by the introperature in the lower part of the furnace substantially above the fusion temperature of the sodium-sulphur com pounds in the ash. The inorganic constituents in kraft pulp liquor are mainly sodium carbonate '(Na CO and sodium sulphate (-Na sO the latter being advantageously reduced to sodium sulfide (Na s) by reaction with the char in the pile on the furnace floor. The molten chemicals trickle down through the char pile to the floor and discharge through a smelt spout at the lower end of the floor to a dissolving tank.

Probably the most important operating condition in recovery furnaces is the furnace temperature. The higher the solids concentration of the incoming black liquor, the higher will be the furnace and char bed temperatures. High furnace temperatures are desirable from the standpoint of maintaining completeness of combustion and furnace stability. High furnace temperatures however are undesirable from the standpoint of cansing sublimation of some of the ash constituents of the liquor, and the resultant condensation of the fume and deposition of chemical ash on the associated steam ..gen-

2,893,829 Patented July 7, 1959 erating and superheating tubes. The gas passages are wholly or partly closed by these deposits and may cause frequent outages of the unit. For this reason the char bed temperature is usually kept as low as possible, even at the risk of an occasional blackout of all or a portion of the char bed. When such a blackout occurs it is customary to promote combustion in that section of the bed by the introduction of either an oxygen lance or the firing of an auxiliary oil burner adjacent that section. Such measures usually result in an excessive local temperature increase which tends to cause sublimation of some of the chemicals in that portion of the bed.

The general object of the present invention is the construction and operation of recovery apparatus of the character described in which the char bed temperature specifically, the object of this invention is the control of char bed temperatures by the regulable introduction of a gas which tends to reduce the temperature of the charbed, and preferably, by the controlled recirculation of relatively cool gaseous products of combustion and the introduction of the recirculated flue gases into the furnace in such a manner as to retard combustion of the char and also to produce an endothermic chemical reaction in the char bed, thus tending to reduce the bed temperature.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described preferred embodiments of the invention.

Of the drawings:

Fig. 1 is an elevation, partly in section, of a black liquor recovery plant constructed in accordance with the invention, some of the equipment being omitted for purposes of clarity;

Fig. 2 is a partly diagrammatic enlarged transverse section of the lower portion of a recovery unit such as shown in Fig. l; and q 7 Fig. 3 is a section taken on the line 3--3 of Fig. 2.

The chemical recovery plant illustrated in Fig. 1 comprises a recovery unit of a type similar to that disclosed in US. Patent 2,594,267, granted April 22, 1952, and comprising a vertically elongated furnace chamber 10 of rectangular horizontal and vertical cross-section, defined by vertical front and rear Walls 11, 12, side walls 13, inclined roof 14, and an inclined floor or hearth 15. A hooded smelt spout 16 at the lower end of the 'fioor is arranged to discharge into'a dissolving tank 17.

Steam generating tubes 18 extend upwardly throughout the height of the furnace front and side walls, while tubes 20 which extend along the floor and rear Wall are bent inwardly and upwardly to define a water tube screen 21 across the exit from the furnace chamber. The lowermost portions of the wall tubes are covered by studs and initially plastic refractory to define a high temperature combustion zone. A spray nozzle 22 is oscillatably mounted in the front wall of the furnace chamber and arranged to variably discharge a sheet of relatively coarse particles of black liquor against the furnace side and rear walls, drying the deposited liquor thereon to a substantially dry char which falls in lumps to the hearth. The char is burned by the addition of preheated combustion air supplied from a forced draft fan 27 through a multi-pass tubular air heater 28, and through the duct work hereinafter described to a series of vertically elongated primaryair portsin the front, side and rear 3 walls and an upper series of secondary air ports 30 in the side and rear walls, subjacent to the area of spray deposition on the walls.

The water screen tubes 21 discharge into a horizontally arranged steam and water drum 32 forming the upper drum of a two-drum steam boiler arranged to .receive the heating gases from the furnace chamber 10. A nose baflie 31 is formed by a row of refractory covered tubes extending inwardly from the rear wall 12 and then upwardly and rearwardly to define a furnace gas outlet. A bank of pendant multiple-looped superheater tubes 34 are positioned above the nose battle for superheating steam from the drum 32. Banks of water tubes 35 and 36 connect the steam and water drum 32 to the lower water drum 37. Vertical baffles 38 and 39 are arranged relative to the tube banks and nose baflie 31 vto define a gas flow successively over the tube banks 35 and 36, and then over a vertical economizer bank 33. .The flue gases after leaving the economizer flow down- .wardly through the tubular air heater 2S and then upwardly through a vertical duct 41 to the upper end of a Venturi type gas and liquid scrubber 43, wherein additional chemicals and heat are recovered, and the gases discharged into a cyclone separator 45. The black liquor separated in the cyclone separator is used as wash liquor in the Venturi scrubber. The gases then pass through a duct 48 to an induced draft fan 43.

As shown in Figs. 2 and 3, the introduction of the concentrated black liquor in a relatively coarse spray results in deposition of substantially all of the liquor on the side and rear furnace walls where it tends to -remain until substantially dehydrated by the heating .efiect of the ascending gas stream and furnace radiation. Lumps of dried char drop from the walls onto the furnace floor and build up as indicated in Figs. 2 and 3. Normally about 60% of the total air required for combustion purposes is supplied through the primary air ports 29, while about 40% is discharged through the secondary air ports. The streams of primary air partly penetrate the char bed and provide a thick layer of in- -candescent char in the bed, with a combustion rate suf- .ficient to maintain an average char bed temperature above the fusion temperature of the incombustible residue of the char. Combustion of any incompletely burned combustible gases rising from the char bed and volatiles from the sprayed liquor is completed by the introduction of the secondary air streams through'the ports 30 and intimate mixing with the gases. The stream of hot flue gases passes successively over the water tube screen, super- 'heater, generating tube banks, economizer and air heater to recover the heat content thereof.

As shown in Fig. 1, the air for combustion'is sup .plied by the forced draft fan 27 and flows across the tubes of the air heater 28 in three horizontal passes, being discharged through vertical duct 54 controlled by dampers 55 into a horizontal duct 56 which opens into a primary air conduit 57 encircling the furnace. 'A pair of separate secondary air conduits 58 controlled by dampers 59 connect to the air heater at the outer sides of the primary air conduits and extend along the rear wall and opposite sides of the furnace chamber, opening vinto the secondary air ports 30. The primary air duct 57 opens to the primary air ports 29. Each primary and secondary air port is individually controlled by a sliding damper 60.

In accordance with the preferred form of the invention flue gases at a relatively low temperature are recirculated to the furnace chamber from a point in the gas ,flow path beyond the economizer 33 and preferably, as shown in Fig. 1, from a point beyond the cyclone separator 45, so that the gases withdrawn will be in their cleanest condition and avoid subsequent deposits on operating elements. The flue gases are withdrawn by a recirculating fan 70 through a pipe 71 connected to the .duct 48 and discharged through a duct 72 controlled by a damper 73 to the supply duct 56 for the primary air ports.

With the described construction and operation theflue gases leaving the recovery unit will normally contain a substantial percentage of carbon dioxide (CO and water vapor (H O). When such gases are brought into intimate contact with carbon in a high temperature reactive condition, an endothermic chemical reaction occurs involving the absorption of a considerable amount of heat. The reactions are:

When recirculated flue gases are introduced with the primary air through the primary air ports as described, the recirculated gas serves first as a diluent at the surface of the bed tending to retard combustion of the char, and secondly as causing the described endothermic reactions. While this results in a substantial reduction of the char bed temperature, the subsequent exothermic reassociation of the gaseous constituents in the secondary air zone to carbon dioxide and water vapor adds to the heat content of the gases, so that little or no net heat loss occurs. During gas recirculation, the amount of primary air is correspondingly reduced, so that the total air-flue gas flow through the primary air ports will be substantially the same. The amount of secondary air is correspondingly increased, so that the total combustion air will be unchanged.

Tests have been made on a pilot recovery unit with 10-20% of the flue gases recirculated. For example, recirculation of 14% of the flue gases resulted in a change in color of the char bed as seen through the primary air ports, from a bright orange to a dull cherry red color, while the gases at the level of the secondary air ports remained a bright orange color, with no loss in furnace stability. Optical pyrometer observations showed a drop in the bed temperature of -150 F. (for example, from 1650-1880 F. to 15501650 F.), a reduction in the smelt temperature of 60 (for example, from 1560 F. to 1500 F.), and an increase in the temperature in the secondary air combustion zone of 50 F. Since the bed temperature is dependent upon the quantity of flue gas recirculated, and that quantity can be changed instantly and easily, the invention permits an easy method of effectively controlling the bed temperatures at any stable level desired. The operator can maintain the bed temperature at the optimum value for minimizing sublimation without fear of blackouts. If and when the char bed should begin to darken, the recirculated gas damper 73 is simply 'moved towards its closed position to improve the combustion conditions in the bed. Automatic regulation of the damper 73 in response to changes in the indication of an optical pyrometer pointed at the char bed can be readily effected. The consequent reduction in deposits on the supenheating and generating tubes reduces the amount of equipment and/or labor required for cleaning the unit during operation. The chemical carryover from the unit is similarly reduced, permitting a reduction in the amount of dust recovery equipment required and the amount of chemical make-up. The desired recirculation of flue gas for control of the char bed temperature by the endothermic reaction of carbon dioxide to carbon monoxide appears to be self-regulating to some extent. A poorly burning bed of char will give oif oxygen-rich flue gas which, when recirculated, tends to increase combustion in the char bed, whereas a brightly burning bed of char will give off flue gas of relatively low oxygen content which in turn tends to retard combustion in the char bed. The described method also permits reasonable adjustments to compensate for pulp mill variables, such as the concentration of black liquor or the quantity of salt cake added to the furnace chamber.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form of the invention now known. to me, those skilled in. the art will understand that changes may be madeinthe form of the apparatus disclosed without departing from the spirit of the invention covered by the claims, and that certain features .of the invention may sometimes be used to advantage without a corresponding use of other features.

' What is claimed is:

l. A continuous self-sustaining, .processof recovering chemicals and heat from pulp residual liquor containing chemicals and combustible organic matter, which comprises p y nstheiiqu rintoap ehea ed turnace i tact with an ascending stream of high temperature heating gases, the liquor spray being so directed as to cause the liquor to remain in the upper section of the furnace in heat absorbing relation with the heatingv gases for an interval sufficient to effect substantially complete dehydration of the liquor and to cause a substantially dry char to deposit and form a char bed on the furnace hearth, supplying a series of spaced streams of primary combustion air under a superatmospheric pressure to the lower section of the furnace below the normal upper level of the char bed for the penetrating and burning of the bed of dry char therein, directing the heating gases generated in the lower section upwardly through the upper section for dehydrating the liquor being sprayed thereinto, supplying secondary combustion air directly to the upper section for the burning therein of any combustible gases distilled from the liquor while in the upper section and any unconsumed combustibles escaping from the lower section, and controlling the average temperature within the char bed on the hearth of said furnace to minimize the sublimation of chemicals, said control being effected by introducing relatively cool gaseous products of combustion mixed with said primary combustion air streams into intimate contact with said char bed to effect an endothermic reducing reaction of said char with at least one of the reducible gaseous constituents of said heating gases.

2. A continuous self-sustaining process of recovering chemicals and heat from pulp residual liquor containing chemicals and combustible organic matter, which comprises spraying the liquor into a preheated furnace in contact with an ascending stream of high temperature heating gases, the liquor spray being so directed as to cause the liquor to remain in the upper section of the furnace in heat absorbing relation with the heating gases for an interval sufiicient to effect substantially complete dehydration of the liquor and to cause a substantially dry char to deposit and form a char bed on the furnace hearth, supplying primary combustion air under a superatmospheric pressure to the lower section of the furnace for the penetrating and burning of the bed of dry char therein, directing the heating gases generated in the lower section upwardly through the upper section for dehydrating the liquor being sprayed thereinto, supplying secondary combustion air directly to the upper section for the burn-ing therein of any combustible gases distilled from the liquor while in the upper section and any unconsumed combustibles escaping from the lower section, withdrawing the heating gases from the upper section of the furnace and reducing their temperature by heat transfer contact With fluid heating tubes, and controlling the average temperature Within the char bed on the hearth of said furnace to minimize the sublimation of chemicals, said control being effected by recirculating relatively cool heating gases after passing over said heating tubes in a series of spaced streams under a superatmospheric pressure into intimate contact with said char bed below the upper surface of said char bed to effect an endothermic reducing reaction of said char with at least one of the reducible gaseous coustituents'of said heating gases.

3. A continuous self-sustaining process of recovering chemicals and heat from pulp residual liquor containing chemicals and combustible organic matter, which comprises spraying the liquor into a preheated furnace in contact with an ascending stream of high temperature heating gases, the liquor spray being so directed as to cause'the liquor to remain in the upper section or the furnace in heat absorbing relation with the heating ga es for an interval sufficient to effect substantially complete dehydration of and at least partial distillation ofzfl e volatile combustibles inthe quo and to cause a su s anti l y char to deposit and form a char .bed on the furnace h a h, s pp yi g a se es a spasedst e m f pr m y ombus n ai und r a. upe a m phe p s to the lower section of the furnace helow-thenormal upper level of the char bed for the penetrating and burning of the bed of dry char therein through ports directed towards :said bed of char, directing the heating gases generated in the lower section upwardly through the upper section for de ydratin e iq o s p ayed t erein o, sup y secondary .combustionair directlyto the upper section for t bur th r in f any mbustible "gas t l e from the liquor while in the-upper section and any unconsumed ,cornbustibles escaping from the lower section, i hdraw g the .heat ng'ga es from the upp tion of he u ac an r dug s thei temper tu e by he t t ansfer contact with fluid" heating tubes, controlling the average temperature within the char bed on the hearth of said furnace to minimize the sublimation of chemicals, said control being effected by recirculating relatively cool heating gases premixed with said primary air streams through said ports into intimate contact with said char bed to effect an endothermic reducing reaction of said char with some of the gaseous constituents of said heating gases, and reoxidizing the reduced gaseous constituents by contact with secondary air.

4. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber having a heating gas outlet in the upper portion thereof, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed in a char bed and the inorganic chemicals are smelted, a series of primary air inlet ports in the vertical walls of said furnace chamlber adjacent said hearth, a series of secondary combustion air ports spaced above said primary air ports, fluid heating tubes arranged to be contacted by the heating gases leaving the upper portion of said [furnace chamber, means for controlling the average temperature within the char bed on the furnace chamber hearth to minimize the sublimation of chemicals comprising a gas recirculating fan, a duct arranged to conduct relatively cool heating gases after passing over said fluid heating tubes to the inlet of said fan, and a second duct connecting the gas outlet side of said fan to said furnace chamber adjacent said hearth and below the normal upper level of said char bed.

5. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated (furnace chamber having a heating gas outlet in the upper portion thereof, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed in a char bed and the inorganic chemicals are smelted, a series of primary air inlet ports in the vertical Walls of said furnace chamber adjacent said hearth, a series of secondary combustion air ports spaced above said primary air ports, fan means for supplying air under a superatmosphen'c pressure to said primary and secondary air ports, fluid heating tubes arranged to be contacted by the heating gases leaving the upper portion of said furnace chamber, means for controlling the average temperature within the char bed on the furnace chamber hearth to minimize the sublimation of chemicals comprising a gas recirculating fan, a duct arranged to conduct relatively cool heating gases after passing over said fluid heating tubes to the inlet of said fan, a second duct connecting the gas outlet 'side of 'said fan to said air ports, and damper means for controlling the flow of recirculated gases.

6. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber of substantially rectangular cross-section and having a heating gas outlet in the upper portion thereof, means [for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed in a char bed. and the inorganic chemicals are smelted, a row of primary air inlet ports in the vertical walls of said furnace chamber adjacent said hearth, a row of secondary combustion air ports spaced above said primary air ports, means for supplying combustion air under a superatmospheric pressure to said primary and secondary air ports, means for regulably proportio'ning said air supply between said primary and secondary air ports, a bank of fluid heating tubes arranged to be contacted bythe heating gases leaving the upper portion of said furnace chamber, means for con- 8 trolling the average temperature-within the char bed on the furnace chamber hearth to minimize the sublimation of chemicals comprising a gas recirculating fan, a duct arranged 'to conduct relatively cool heating gases after passing over said fluid heating tubes to the inlet of said fan, a second duct connecting the gas outlet side of said fan to said primary air ports, and damper means in said second duct for controlling the flow of recirculated gases.

References Cited in the file of this patent UNITED STATES PATENTS 1,699,443 Owen Jan. 15, 1929 1,771,829, Wagner July 29, 1930 1,896,910 Merkt Feb. 7, 1933 1,912,621 Clark June 6, 1933 1,931,536 Goodell Oct. 24, 1933 1,933,255 Goodell Oct. 31, 1933 2,198,446 Wilcoxson Apr. 23, 1940 2,258,401 Badenhausen Oct. 7, 1941 2,594,267 Wilcoxson Apr. 22, 1952 2,602,022 Patterson July 1, 1952 

1. A CONTINUOUS SELF-SUSTAINING PROCESS OF RECOVERING CHEMICALS AND HEAT FROM PULP RESIDUAL LIQUOR CONTAINING CHEMICALS AND COMBUSTIBLE ORGANIC MATTER, WHICH COMPRISES SPRAYING THE LIQUOR INTO A PREHEATED FURNACE IN CONTACT WITH AN ASCENDING STREAM OF HIGH TEMPERATURE HEATING GASES, THE LIQUOR SPRAY BEING SO DIRECTED AS TO CAUSE THE LIQUOR TO REMAIN IN THE UPPER SECTION OF THE FURANCE IN HEAT ABSORBING RELATION WITH THE HEATING GASRS FOR AN INTERVAL SUFFICIENT TO EFFECT SUBSTANTIALLY COMPLETE DEHYDRATION OF THE LIQUOR AND TO CAUSE A SUBSTANTIALLY DRY CHAR TO DEPOSIT AND FORM A CHAR BED ON THE FURANCE HEARTH, SUPPLYING A SERIES OF SPACED STREAMS OF PRIMARY COMBUSTION AIR UNDER A SUPERATMOSPHERIC PRESSURE TO THE LOWER SECTION OF THE FURNACE BELOW THE NORMAL UPPER LEVEL OF THE CHAR BED FOR THE PENETRATING AND BURNING OF THE BED OF DRY CHAR THEREIN, DIRECTING THE HEATING GASES GENERATED IN THE LOWER SECTION UPWARDLY THROUGH THE UPPER SECTION FOR DEHYDRATING THE LIQUOR BEING SPRAYED THEREINTO, SUPPLYING SECONDARY COMBUSTION AIR DIRECTLY TO THE UPPER SECTION FOR THE BURNING THEREIN OF ANY COMBUSTIBLE GASES DISTILLED FROM THE LIQUOR WHILE IN THE UPPER SECTION AND ANY UNCONSUMED COMBUSTIBLES EXCAPING FROM THE LOWER SECTION, AND CONTROLLING THE AVERAGE TEMPERATURE WITHIN THE 